This research program will explore the following hypotheses: (1)\Natural cell-mediated cytotoxicity (NCMC) is a complex phenomenon which is mediated by more than one mononuclear cell subset and is modulated by cellular interaction. (2)\NCMC is regulated by interferon and other soluble factors through differential effects on the various effector cell types to produce the resultant natural cytotoxic effects. In order to test these hypotheses, we propose: (1)\to identify and isolate the cell types involved in NCMC, using physical and biological criteria; (2)\to define the roles and interactions of these lymphocyte subpopulations in the regulation of NCMC, and to determine the importance of similar mechanisms in ADCC; and (3)\to elucidate the roles of soluble mediators including interferon in the regulation and modulation of NCMC. Two cytotoxicity tests, chromium release and single cell assays, are used to study NCMC. Lymphocytes are separated by density and size, as well as by methods using cell surface markers. Subpopulations thus obtained are mixed in different combinations to define interactions in cytotoxic reactions. Lymphocyte subsets are treated with interferon and other factors, and mixture experiments will be performed on treated and untreated combinations to understand the control mechanisms such factors exert on NCMC. Two populations of NCMC effectors which can be isolated by density have been identified. The LGL population sediments toward the top of the Percoll density gradient. A heavier population is identified by a smaller increase in activity at the bottom of the gradient. These populations may differ in their recycling abilities. NCMC effector cells self-regulate by producing interferon. Accessory and helper cells are being investigated.